Basic support to track the backfilling of repositories
What does this MR do?
- Remove the need to trigger the backfilling mechanism from the primary node.
- Creates a secondary database where secondaries nodes (read-only) can write to.
- Implements a tracking mechanism to backfill the repositories.
- Attempts to backfill one repo at a time.
- Primary node can disable/enable activity on the secondaries. Database replication will still happen since that is handled by PostgresSQL itself and not by the Rails application.